The present invention relates to an optical element mainly used in an optical communication system, an optical measuring machine or the like. Particularly it relates to an optical element using photonic crystal.
It is known well that photonic crystal having a structure in which dielectrics different in refractive index are arranged periodically at intervals of a period substantially equal to the wavelength of light has the following characteristic properties:
(a) confinement of light due to a photonic band gap;
(b) very large wavelength dispersion due to a unique band structure; and
(c) abnormality in group velocity of propagated light.
A large number of optical elements using such properties have been proposed or examined.
Photonic crystal can be classified into one-dimensional photonic crystal, two-dimensional photonic crystal and three-dimensional photonic crystal by the number of directions having periodic structures. For example, the simplest one-dimensional photonic crystal is a filter formed in such a manner that two kinds of thin films (e.g., SiO2 films and TiO2 films) are laminated alternately on a parallel-plane substrate. The filter has been already put into practical use. This structure has a function of reflecting only input light in a specific wavelength range because it has a photonic band gap in a periodic direction. In addition, because the wavelength range of the photonic band gap for oblique input light varies in accordance with the direction of polarization, the filter can be provided to function as a polarized light separating filter.
A structure formed by application of a photolithography technique in such a manner that air holes are arranged in thin films on a substrate has been examined widely as the two-dimensional photonic crystal. If a linear defect is formed in the arrangement of air holes, the portion of the linear defect can be provided as an optical waveguide.
Because a steric optical waveguide can be achieved if a photonic band gap is achieved in all directions in the three-dimensional photonic crystal, there is expectation that a large number of optical elements will be integrated into an about 1 mm cube.
Of the one-dimensional, two-dimensional and three-dimensional photonic crystals, the one-dimensional photonic crystal has been not investigated so sufficiently as the two-dimensional and three-dimensional photonic crystals because the means of making the best use of the properties of the one-dimensional photonic crystal is almost limited to the aforementioned filter though the one-dimensional photonic crystal has a large merit that the one-dimensional photonic crystal can be produced easily. The properties (b) and (c) of the one-dimensional photonic crystal, however, can be utilized sufficiently though the one-dimensional photonic crystal is inferior in the property (a) to the two-dimensional and three-dimensional photonic crystals. As the means using the properties (b) and (c), there is an example in which an end surface of a multilayer film, that is, a surface having a multilayer structure exposed is used as a light input surface or a light output surface.
For example, theoretical analysis of directions of light rays incident onto a section of an inclined multilayer film has been described in Applied Physics B, Vol.39, p.231, 1986. There has been also disclosed a technique for obtaining the same polarized light separating effect as that of a birefringent material by using the property (so-called structural birefringence) of a multilayer film in which the refractive index of the multilayer film varies widely in accordance with TE/TM polarized light, aiming at separating polarized light by structural birefringence (Optics Letters Vol.15, No.9, p.516, 1990). There has been a further report that very large dispersion (super-prism effect) is obtained because the shape of the first photonic band of a periodic multilayer film is linear in proximity to a band gap (xe2x80x9cInternational Workshop on Photonic and Electromagnetic Crystal Structuresxe2x80x9d Technical Digest, F1-3).
According to the inventor""s examination, it has been further made clear that light substantially perpendicularly incident onto an end surface of a multilayer film is propagated in an aperiodic direction so that the characteristic of photonic crystal can be brought out. According to the inventor""s electromagnetic wave simulation and experiment, when plane wave of monochromatic light is made substantially perpendicularly incident onto an end surface of a one-dimensional photonic crystal (periodic multilayer film), the light is separated into waves corresponding to some bands so that the waves are propagated through the multilayer film. When the wavelength of the input light is sufficiently long compared with the period of the multilayer film, only a wave corresponding to the first band (hereinafter referred to as xe2x80x9cfirst band lightxe2x80x9d) is propagated. As the wavelength of the input light becomes shorter, high-order waves such as third band light and fifth band light begin to propagate successively. Accordingly, a part of energy of the input light always propagates as first band light regardless of the wavelength of the input light.
The high-order band light such as third band light or fifth band light has the properties (b) and (c) whereas the first band light does not have the properties (b) and (c). Accordingly, the first band light is wasteful light which is almost useless for an optical element. There is a problem that the first band light serves as stray light causing lowering of an S/N ratio of the element as well as the first band light reduces the efficiency of utilizing input light.
The invention is provided to solve this problem and an object of the invention is to provide an optical element having a means for propagating only specific high-order band light through photonic crystal.
In the invention, light incident onto an end surface of one-dimensional photonic crystal is phase-modulated in the same period and direction as those of the photonic crystal to thereby propagate only specific high-order band light through the photonic crystal. Or light emergent from an end surface of the photonic crystal is phase-modulated so as to be converted into plane wave.
The action is achieved by the following means.
An optical element according to the invention includes a multilayer structure containing repetition of a periodic structure as at least one repeatable region of a predetermined period, an end surface of the multilayer structure substantially perpendicular to layer surfaces of the multilayer structure being used as a light input surface. The optical element configured as described above further has a phase modulation unit (phase modulation means) disposed adjacent or close to the light input surface for generating phase-modulated wave having the same period as the period of the periodic structure in a laminating direction of the multilayer structure.
Preferably, an end surface of the multilayer structure substantially perpendicular to layer surfaces of the multilayer structure and opposite to the light input surface is used as a light output surface, and the optical element further has a phase modulation unit disposed adjacent or close to the light output surface for substantially converting output light of the multilayer structure into plane wave.
The periodic structure can be regarded as one-dimensional photonic crystal. The phase-modulated wave is generated in such a manner that wave belonging to a single associative photonic band except the lowest-order band is propagated mainly in a direction perpendicular to the periodic direction and not containing the periodic structure.
In the optical element according to the invention, nearly plane wave from the outside of the multilayer structure is converted into phase-modulated wave by the phase modulation unit so that the phase-modulated wave is input into the multilayer structure. The phase modulation unit is constituted either by a phase grating having the same period as that of the periodic structure or by an optical system for making a plurality of plane-wave light rays of the same frequency interfere with one another. Preferably, the phase grating is integrated with the periodic structure. The phase grating can be formed as apart of the multilayer structure separated by a groove formed in proximity to either the light input end surface or the light output end surface of the multilayer structure. The phase modulation unit disposed on the light output side can be formed in the same manner as described above.
The present disclosure relates to the subject matter contained in Japanese patent application No. 2002-12334 (filed on Jan. 22, 2002), which is expressly incorporated herein by reference in its entirety.